Blood transfusion is a central therapeutic aid in modem medicine. It is a primary treatment in the field of emergency medicine. For this reason, since the beginning of the twentieth century, blood has been collected and stored in blood banks. Initially collected and stored as whole blood, blood which is obtained from donors is now separated into defined fractions before storage and eventual use to treat patients. These blood components are stored in closed plastic bags at temperatures ranging from −80÷C to +24÷C, depending upon the fraction. Table 1 below gives the current storage conditions of blood fractions.
TABLE 1Storage conditions of blood componentsComponentDurationTemperature, ° C.Whole blood24 hours20-24Red cells42 days4Platelets3-5 days20-24Stem cells10 years−180PlasmaSeveral years−20
Although human blood is distributed internationally, maintaining an adequate supply depends upon a number of factors, including the availability of donors, the provision of suitable collection and storage facilities, and the limited shelf-life of this biological material. Therefore, the medical community is interested in developing new procedures for extending the shelf-life of these blood components.
Theoretically, the shelf-life of preserved blood components depends upon two major factors: the time period during which the function of the blood components can be maintained in storage, and the reduction of pathogen contamination. The extended maintenance of the function of these components in storage has been achieved by adding such materials as phosphates and/or compounds to arrest undesirable biological activity such as coagulation, changing the pH balance of the storage medium, and maintaining the proper temperature for the particular component (as described with regard to Table 1 above). For certain types of blood components, reduced temperature levels are suitable for storage and also help to reduce the rate of the growth of contaminating microorganisms. However, for other components, such as platelets, reduced temperature levels may induce a loss of biological function and therefore cannot be used to reduce pathogen contamination.
Contamination of blood by pathogens has long been recognized as a significant complication of blood transfusion. Even if healthy donors are selected, and the resultant donated blood is screened for the presence of various types of pathogens, including viruses such as hepatitis and HIV, blood components which are stored for an extended period of time are vulnerable to pathogen contamination.
In order to help reduce such contamination, blood is collected from donors under aseptic conditions. Sterile closed systems are used for the collection and processing of blood components, further reducing pathogen contamination. However, the presence of bacteria in blood components is still currently the most common microbiological cause of transfusion-associated morbidity and mortality. Transfusion-associated contamination which is caused by the inadvertent intravenous infusion of pathogen contaminated platelets appears to be much more common than complications caused by contamination of red blood cells or plasma. This may be due to the fact that significant morbidity and mortality occurs when the contaminated blood product contains a sufficiently large number of bacteria (≧106), thereby resulting in a relatively high level of bacterial endotoxins. Since platelets cannot be stored at temperatures lower than 20° C. without risking the loss of biological function, the risk of contamination is proportionally much larger with platelets than with red blood cells. Indeed the rate of reported complications from infected platelets is greater than that of red blood cells by a 2:1 ratio.
Platelets are enucleated cells derived from bone marrow megakaryocytes. They play an important role in homeostasis, blood clotting and thrombosis. The life span for platelets in the blood circulation of the body is estimated to be about ten to twelve days. However, after five to six days of ex-vivo storage, platelets age, as evidenced by morphological signs of apoptosis such as a change in shape from discoid to spherical, and the presence of membrane blebbing. Another measurable parameter for platelet viability is the pH of the surrounding buffer; when it falls below pH 6.0, viability is lost. Another measurement of platelet viability is the leakage of enzymes. In particular, leakage of LDH (lactic dehydrogenase) is used as a parameter for loss of platelet viability.
Various studies have confirmed that pathogen contamination of platelets causes the highest level of mortality of all the different blood components and products. For allogeneic transfusions, the mortality rate for apheresis platelet transfusion was seven times higher than the risk of an adverse event following platelet concentrate transfusion, and more than three times higher than the risk following red blood cell transfusion. The risk increases to twelve times higher after platelet pool transfusion (from multiple donors) and 5.5 times higher after apheresis platelet concentrate infusion (all statistics from Perez et al., “Determinants of transfusion associated bacterial contamination; Results of the French BACTHEM Case Control Study”; Transfusion, 2001, vol 41, pp. 477-482; see also K. Sazama, “Bacteria in Blood for Transfusion: A Review”, Arch Pathol Lab Med, vol 118, 1994, pp. 350-365). These studies of the risk of platelet contamination has led to the shortening of their shelf life from 7 to 5 days by the FDA in 1986. However, this short time window effectively reduces available supplies of platelets.
The medical community is therefore currently considering two options: providing blood banks with more rapid bacterial screening methods; and developing methods for the control of growth of bacteria and/or other pathogens. The former approach has a number of drawbacks, including lower sensitivity of the more rapid bacterial detection methods and increased expense. The latter approach has been explored, generally involving the destruction of the ability to replicate genetic material, as this approach is believed to be safe for the enucleated blood cells like red cells and platelets. For example, cross-linking chemicals, with and without the requirement for photoactivation, have been considered. Examples of such chemicals include psoralens 8-MOP, AMT and most recently, S-59 “inactine”. These chemicals are considered to be hazardous to the human body and thus must be removed post-treatment, before the platelets can be given to a patient. Current removal methods include filtration or washing protocols in order to remove agents which are not bound in some manner to the surface of cells or proteins.
Since the removal process is time consuming and may also damage the blood cells, other less hazardous, agents have been considered. One example of such an agent is riboflavin, which upon photoactivation forms lumichrome. However, this agent has been shown to have variable effectiveness for bacterial inactivation and may even decrease platelet survival rates in autologous transfusions performed in primates, which has negative implications for its utility in promoting increased platelet storage times (“Connect with Safer Blood Products: Abstracts on Pathogen Eradication Technology”, Gambro BCT Inc., USA, 2001).
None of the above preservation methods has been approved yet. Each method has a number of disadvantages, including the fact that they are often suitable only for one blood component, and that the removal methods employed may damage the blood components. Also, the preservation method itself may damage the blood components. Therefore, there is currently no suitable method for preservation of whole blood and/or blood components, which does not involve the introduction of potentially hazardous chemicals into the human body, and which does not damage the whole blood and/or blood components themselves.
Carbon monoxide (CO) is a natural product of hemo-proteins degradation in the human body and chemically inert. It is known as a highly toxic gas due to its ability to replace, with high affinity, the sites for oxygen in hemoglobin. However, a growing body of scientific evidence has indicated in the last decade that the same molecule serves also basic physiological roles like transmission in the neurological system. Thus its location and quantity appears to determine whether carbon monoxide is helpful or harmful to the body.